When immune to suffocation, it can mean one of several things, though all with the same effect, game wise. Either the person has a mask or other air supply that means he doesn't need to worry about breathing the air around him, or he has no need to breath, like a vampire. Either way, he could avoid gas attacks that require the gas to be inhaled simply by either using his alternate air supply, or not breathing. But gas attacks that cause damage on contact, or don't need to be inhaled, would still get by it. Real world equivalents of things that would get past immunity: suffocation would be things like mustard gas, which is highly corrosive, or some nerve gasses, which only need contact with the skin to work. Immunity to suffocation would not be a sure defense against biological weapons either.